In a desolate future, children are kept away from the outside world, away from the open sky. Locked away in Mistilteinn, aka the ‘birdcage’, they live only to pilot giant mechs known as FranXX. Hiro was once a prodigy but after falling behind, he’s become unnecessary. After all, not piloting a FranXX is the same as not existing. But everything changes when he meets Zero Two, the girl with horns.